Due to the unique problems of space exploration, different power sources had to be developed for powering the many electrical components which are utilized in space rockets, capsules or other devices. Many of these power sources contained a battery having a number of nickel-cadmium sintered plate cells, since these cells have a high energy density and can be recharged. A primary factor in the power system that used a nickel-cadmium battery is the capacity degradation of the batteries. Since obviously, these batteries while used in space cannot be physically replaced, an approach must be developed which would extend the capacity life by periodically reconditioning the batteries. A prior method of reconditioning these batteries has been accomplished by using relays to individually control the discharge of each cell. However, it has been determined that this method is not practical for inflight applications because of the bulk and complexity of using a relay, cell sensing and relay control circuitry for each individual cell.
In developing a method which can be utilized in inflight applications, it should be remembered that the voltage of a nickel-cadmium cell should not be reversed during a discharge since there is a chemical reaction which produces hydrogen gas. This gas can build up considerable pressure in a sealed cell or cause a vented cell to emit the hydrogen gas. Either situation can be detrimental to the battery plus hazardous to any personnel which are in the area. Since these cells are never identical, there will always be some cells which would go through zero and reverse before other cells are discharged. Therefore, a method must be developed which prevents the cell reversal of the nickel-cadmium battery.
The need for reconditioning a nickel-cadmium battery is important since it has been demonstrated that as the result of "memory" or "fading" the effective energy capacity of the nickel-cadmium battery can be decreased by as much as 60-80% under operating conditions.
A review of the prior art indicates that while it is old to increase the life of a nickel-cadmium battery by reconditioning, no prior art reference was discovered which efficiently monitors each cell of the battery for preventing cell reversal. A typical prior art reference is British patent 1,256,980 issued to Brinsley Hodge. This patent is directed to a method for restoring the electrical capacity of nickel-cadmium cells by subjecting each cell to a complete discharge by connecting a resistance means across its electrodes. Additional prior art devices include British patent 864,844 issued to John Jephcott and French patents 1,203,578 and 1,217,578. However, none of these references disclose a device which efficiently monitors each cell of a nickel-cadmium battery to prevent the cells of the battery from reversing polarity.